The DS409 has all of the features of the DS409+, but uses a Marvell "Kirkwood" storage processor and has 256 MB of RAM. The DS409+ runs on a Freescale MPC8533 and has 512 MB of RAM.
SmallNetBuilder's testing revealed similar Vista SP1 file copy write performance to RAID 5 volumes measuring 34 MB/s for the DS409+ and 31 MB/s for the DS409. The difference in read file copy performance was much more dramatic, measuring 70 MB/s for the DS409+ and 57 MB/s for the DS409.
The DS409 is available now at an MSRP of $439 vs. $599 for the DS409+.